Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) are playable by optical storage devices. When an optical pickup head of an optical storage device operates, the light emitted by a light source such as a laser diode is focused by an object lens of the optical pickup head on an optical disk, and the light reflected by the optical disk is transmitted to a light sensor to realize information from the disk. Referring to FIG. 1, the optical pickup head 10 moves along two main directions, i.e. a direction perpendicular to the disk surface, referred as a focusing direction F, and a direction parallel to the disk surface, referred as a tracking direction T.
During operation, a focusing error and a tracking error usually arise on the rotating disk. The focusing error is generally caused by vibration of the disk in the focusing direction F, and the tracking error is caused by eccentricity of the disk in the tracking direction T. To correct these errors, a tracking control system for an optical pickup head was developed, as can be seen in FIG. 2. The optical sensor 20 of the optical pickup head has six light receiving parts A, B, C, D, E and F for producing output signals, when receiving light reflected from the optical disk. The output signals e and f are processed through an operational amplifier 21 to produce a tracking error signal TE. An ideal tracking error signal TE1 would be an alternating current (AC) signal distributing in a preset amplitude range without any direct current (DC) component effect, as shown in FIG. 3(a). That is, the DC level of the AC signal is preferably zero. The waveform of the tracking error signal TE1 is symmetrical with respect to the zero level 0. It is known to those skilled in the art that the zero level, which indicates the best tracking condition, is an index for performing a feedback control.
In practice, however, the tracking error signal generated through the optical sensor 20 and operational amplifier 21 may involve non-zero DC level, as exemplified by the signal TE2 shown in FIG. 3(b). As is known, the effect of DC component varies with conditions such as resolving speed, tracking position or quality of disc. Once the tracking error signal TE fluctuates, error situation of the tracking error zero cross (TEZC) signal derived from the tracking error signal TE may occur so as to further adverse the seeking operation of the optical pickup head. For example, the optical pickup head is supposed to jump from an outer track of a disc to an inner track in a seeking operation according to the TEZC signal. If the TEZC signal referred by the optical pickup head is erroneous in the meantime, the optical pickup head may wrongly jump further outwards from the outer track so as to read wrong information. Further, if the starting outer track of that seeking operation is near the outer edge of the disc, the outward jumping may result in jumping out of the disc range and even hitting other parts of the optical disc drive. Therefore, it is desirable to modify the tracking error signal TE2 in order to eliminate the non-zero DC component, thereby stabilizing the TE signal, obtaining accurate TEZC signal and precisely moving the optical pickup head. Currently, to input an offset voltage into the operational amplifier 21, as shown in FIG. 2, is one of the accesses to adjust the TE signal.
Conventionally, the level of the offset voltage is determined as soon as the initial calibration of the optical disc drive is performed. Once the offset voltage has been determined according to certain calculation, the constant offset voltage is inputted to the operational amplifier 21 to modify the tracking error signal TE in all following procedures. In other words, no matter how the seeking or tracking conditions change, the offset voltage is kept the same. Therefore, the conventional TE-signal adjustment using a constant offset voltage is not precise enough for obtaining a reliable TE signal. Moreover, an improper offset voltage may further adversely affect the seeking operation due to the resulting asymmetric waveform of the TE signal.